AC/DC power converters are often required to convert AC power into DC power for a load. Where the AC power is produced by a generator, it has been found that the size of the generator for a given power requirement by the load can be minimized by operating the converter such that it appears to the generator as a unity power factor load. Numerous controls for operating an AC/DC converter at unity power factor have been developed. For example, an article entitled "Indirect Current Control of a Unity Power Factor Sinusoidal Current Boost Type Three-Phase Rectifier" by Dixon et al. appearing in IEEE Transactions on Industrial Electronics, Vol. 35, No. 4, Nov. 1988, discloses in FIG. 1 a control for a hysteresis current controlled rectifier which maintains the output voltage of the rectifier at a desired level and which causes the converter to operate at unity power factor. The control includes circuitry which develops an amplified and limited voltage error signal representing the deviation of the output voltage of the converter from the desired level, a multiplier which multiplies the error signal with the phase voltages produced by an AC source coupled to the rectifier and summers which sum the output of the multipliers with signals representing the phase currents. The resulting current error signals are coupled to a base drive control logic circuit which develops base drive signals for switches in the converter.
Tanahashi, U.S. Pat. No. 4,816,985 discloses in FIG. 8 thereof an acknowledged prior art control for an AC/DC converter which causes the converter to operate at unity power factor. The control includes a circuit which develops a voltage error signal representing the deviation of the output voltage of the converter from a desired voltage, first through third multipliers which multiply each of first through third voltage waveforms developed by an oscillator with the voltage error signal and first through third current control amplifiers which receive the outputs of the first through third multipliers, respectively. The current control amplifiers calculate and amplify deviations between the signals developed by the multipliers and the outputs of current detectors which develop signals representing the phase currents of an AC source coupled to the converter. The outputs of the current control amplifiers are compared against a sawtooth waveform to develop pulse-width modulation (PWM) base drive signals for switches in the converter.
Other unity power factor converter controls which develop PWM base drive signals for switches of a converter are disclosed in the above-identified Dixon et al paper and Tanahashi patent. These controls develop the base drive signals using voltage signals representing the voltages produced by the AC source coupled to the converter input, as well as signals representing the voltage at the output of the converter and the currents supplied by the AC source.
Wester, U.S. Pat. No. 4,193,111 discloses a controller for an AC/DC power converter which controls switches in the converter to cause the current delivered by an AC source coupled to the converter to track or follow the voltage produced thereby. The control circuit includes first and second comparators which compare a current command against the actual current provided by the AC source. One of the comparators has a narrow hysteresis characteristic and the other comparator has a wide hysteresis characteristic and produces a signal identifying the polarity of the voltage. The outputs of the comparators are coupled to logic circuits including logical inverters and OR gates which together develop signals for switches in the converter.
A control device for an AC/DC power converter which utilizes PWM gating circuitry responsive to the voltage produced by an AC source coupled to the converter and the output voltage of the converter is disclosed in Sato, U.S. Pat. No. 4,729,082.